Mass spectrometric determination of derivatized methylmalonic acid

ABSTRACT

The invention relates to the detection of methylmalonic acid (MMA). In a particular aspect, the invention relates to methods for detecting derivatized methylmalonic acid (MMA) by mass spectrometry.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 13/156,307, filed Jun. 8, 2011, which claims the benefit of U.S.Provisional Appl. No. 61/353,172, filed Jun. 9, 2010, each of which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to the quantitative measurement of methylmalonicacid. In a particular aspect, the invention relates to methods forquantitative measurement of methylmalonic acid by gaschromatography-tandem mass spectrometry.

BACKGROUND OF THE INVENTION

Methylmalonic acid (MMA) is a C-methylated dicarboxylic acid derivativeof malonate. The coenzyme A-linked form of MMA is a metabolicintermediate in the conversion of propionyl-CoA to succinyl-CoA. VitaminB₁₂ is a vital co-factor in this conversion. If insufficient vitamin B₁₂is available, the conversion does not proceed to completion. Build-up ofincomplete conversion products (including L-methylmalonyl-CoA)ultimately leads to increased MMA production. Thus, elevated levels ofMMA are observed when there is a vitamin B₁₂ deficiency.

Several methods for MMA determination in biological samples have beenreported. For example, gas chromatography-single mass spectrometrytechniques utilizing derivatization of MMA prior to analysis have beenreported by Allen et al. (U.S. Pat. Nos. 5,438,017 and 5,457,055;detection of t-butyldimethylsilyl ester derivatives); Lou et al. (J.Huazhong University of Science and Technology (2004) 24:166-9; detectionof N, o-Bis (trimethylsilyl) trifluoroacetamide derivatives); Stabler etal. (J. Clin. Invest. (1986) 77:1606-12; detection oft-butyldimethylsilyl ester derivatives); Marcell, et al. (AnalyticalChem. (1985) 150:58-66; detection of t-butyldimethylsilyl esterderivatives); McCann, et al. (Clin. Chem. (1996) 42:910-14; detection oft-butyldimethylsilyl ester derivatives); Windelberg, et al. (Clin. Chem.(2005) 51:2103-09; detection of methylchloroformate derivatives);Specker, et al. (Am J Clin Nutr. (1990) 51:209-11; detection ofdicyclohexyl ester derivatives); Yazdanpanah, et al. (Clin Biochem.(2003) 36:617-20; detection of N, o-Bis (trimethylsilyl)trifluoroacetamide derivatives); Purevsuren, et al. (Brain and Develop.(2008) 30:520-26; detection of bis(trimethylsilyl)triflurooacetamide andtrimethylchlorosilane derivatives); Chen et al. (Clinical Applicationsof Mass Spectrometry, Methods in Molecular Biology, Chapter 35:365;detection of trimethylchlorosilane and butyl ester derivatives); andYoung, et al. (Analyst (1995) 120:2199-2201; reporting butyl esterderivatives). Liquid chromatography-tandem mass spectrometry techniquesutilizing derivatization of MMA prior to analysis have been reported byShushan et al. (U.S. Pat. No. 6,692,971; detection of n-butyl esterderivatives); Kushnir et al. (U.S. Publication No. 2004/0165560;detection of n-butyl ester derivatives); and Kushnir et al. (Clin. Chem.(2001) 47:1993-2000; detection of dibutyl ether derivatives). Gaschromatography-tandem mass spectrometric techniques have been reportedby Hagen, et al. (Clinica Chimica Atca (1999) 283:77-88; detection oftrimethylsilyl derivatives); and Ueland et al. (Clin Chem Lab Med (2007)445:1737-45; detection of methylchloroformate derivatives).

SUMMARY OF THE INVENTION

One aspect of the present invention provides methods for determining theamount of methylmalonic acid (MMA) in a sample by tandem massspectrometry. In some embodiments, the methods include: (a) obtaining asample containing MMA; (b) subjecting the MMA containing sample to aderivatizing agent under conditions suitable to generatetert-butyldimethylsilyl derivatized MMA (TBDMS-MMA); (c) subjectingTBDMS-MMA from the sample to chromatography; (d) ionizing TBDMS-MMAfollowing chromatography to generate two or more ions detectable by massspectrometry, these ions including a precursor ion with a mass to chargeratio of 289.0±0.5 and one or more fragment ions selected from the groupconsisting of fragment ions with mass to charge ratios of 189.0±0.5,147.0±0.5, and 73.0±0.5; and (e) determining the amount of one or morefragment ions by tandem mass spectrometry. In these methods, the amountof ions determined in step (e) is related to the amount of MMAoriginally present in the sample. In some embodiments, chromatography isgas chromatography (GC). In some embodiments, the derivatizing agentcomprises methyl-(t-butyldimethylsilyl)trifluoroacetamide (MTBSFTA). Insome embodiments, MMA originally present in the sample is purified bysolid phase extraction prior to being subjected to the derivatizingagent. In some embodiments, the sample is from a human patient.

In some embodiments, the methods include: (a) obtaining a samplecontaining MMA; (b) subjecting said sample to a derivatizing agent underconditions suitable to generate tert-butyldimethylsilyl derivatized MMA(TBDMS-MMA); (c) subjecting TBDMS-MMA from the sample to gaschromatography (GC); (d) ionizing TBDMS-MMA following chromatography togenerate two or more ions detectable by mass spectrometry, the ionsincluding a precursor ion and one or more fragment ions; and (e)determining the amount of one or more of the fragment ions by tandemmass spectrometry. In these embodiments, the amount of ions determinedin step (e) is related to the amount of MMA originally present in thesample. In some embodiments, the precursor ion has a mass to chargeratio of 289.0±0.5. In some embodiments, one or more fragment ions areselected from the group consisting of fragment ions with mass to chargeratios of 189.0±0.5, 147.0±0.5, and 73.0±0.5. In some embodiments, thederivatizing agent comprisesmethyl-(t-butyldimethylsilyl)trifluoroacetamide (MTBSFTA). In somerelated embodiments, MMA in the sample is purified by solid phaseextraction prior to being subjected to the derivatizing agent.

In some embodiments, the methods include: (a) obtaining a samplecontaining tert-butyldimethylsilyl derivatized MMA (TBDMS-MMA), whereinTBDMS-MMA is produced by derivatizing MMA originally present in thesample; (b) subjecting TBDMS-MMA from the sample to chromatography; (c)ionizing TBDMS-MMA following chromatography to generate two or more ionsdetectable by mass spectrometry, the ions including a precursor ion witha mass to charge ratio of 289.0±0.5 and one or more fragment ionsselected from the group consisting of fragment ions with mass to chargeratios of 189.0±0.5, 147.0±0.5, and 73.0±0.5; and (d) determining theamount of one or more of the fragment ions by tandem mass spectrometry.In these embodiments, the amount of ions determined in step (d) isrelated to the amount of MMA originally present in the sample. In someembodiments, chromatography is gas chromatography. In some embodiments,the methods further include obtaining a sample containing MMA andsubjecting the MMA containing sample to a derivatizing agent underconditions suitable to generate tert-butyldimethylsilyl derivatized MMA(TBDMS-MMA). In some related embodiments, the derivatizing agentcomprises methyl-(t-butyldimethylsilyl)trifluoroacetamide (MTBSFTA). Insome related embodiments, the sample is purified by solid phaseextraction prior to being subjected to said derivatizing agent.

In a second aspect, methods are presented for determining the amount oftert-butyldimethylsilyl derivatized MMA (TBDMS-MMA) in a sample bytandem mass spectrometry. These methods include: (a) subjectingTBDMS-MMA in the sample to chromatography; (b) ionizing TBDMS-MMAfollowing chromatography to generate two or more ions detectable by massspectrometry, these ions including a precursor ion with a mass to chargeratio of 289.0±0.5 and one or more fragment ions selected from the groupconsisting of fragment ions with mass to charge ratios of 189.0±0.5,147.0±0.5, and 73.0±0.5; and (c) determining the amount of one or moreof the fragment ions by tandem mass spectrometry. In these methods, theamount of ions determined in step (c) is related to the amount of MMAoriginally present in the sample. In some embodiments, chromatography isgas chromatography.

In a third aspect, methods are presented for diagnosing vitamin B₁₂deficiency in a patient. These methods include: (a) obtaining a samplecontaining MMA from a patient; (b) subjecting MMA from the sample to aderivatizing agent under conditions suitable to generatetert-butyldimethylsilyl derivatized MMA (TBDMS-MMA); (c) subjecting theTBDMS-MMA from step (b) to chromatography; (d) ionizing TBDMS-MMAfollowing chromatography to generate two or more ions detectable by massspectrometry, these ions including a precursor ion with a mass to chargeratio of 289.0±0.5 and one or more fragment ions selected from the groupconsisting of fragment ions with mass to charge ratios of 189.0±0.5,147.0±0.5, and 73.0±0.5; (e) determining the amount of one or more ofthe fragment ions by tandem mass spectrometry; (f) determining theamount of MMA originally in the sample from the amount of ionsdetermined in step (e); and (g) diagnosing a vitamin B₁₂ deficiency inthe patient from the amount of MMA determined in step (f). In someembodiments, chromatography is gas chromatography. In some embodiments,the derivatizing agent comprisesmethyl-(t-butyldimethylsilyl)trifluoroacetamide (MTBSFTA). In someembodiments, MMA in the sample is purified by solid phase extractionprior to being subjected to said derivatizing agent.

In some embodiments, TBDMS-MMA is ionized by electron impact ionization.

In some embodiments, the sample is a biological sample, preferably abody fluid sample; for example, plasma or serum, such as from a humanpatient.

In the methods described herein, mass spectrometry is tandem massspectrometry. Tandem mass spectrometry may be conducted by any methodknown in the art, including for example, multiple reaction monitoring,precursor ion scanning, or product ion scanning.

As used herein, “derivatizing” means reacting two molecules to form anew molecule. Thus, a derivatizing agent is an agent that may be reactedwith another substance to derivatize the substance. For example,methyl-(t-butyldimethylsilyl)trifluoroacetamide (MTBSFTA) is aderivatizing reagent that may be reacted with MMA to formtert-butyldimethylsilyl derivatized MMA (TBDMS-MMA).

As used here, the names of derivatized forms of MMA include anindication as to the nature of derivatization. For example,tert-butyldimethylsilyl derivatized MMA is indicated as TBDMS-MMA (orTBDMS-derivatized MMA).

In certain embodiments, mass spectrometry is performed in positive ionmode.

Alternatively, mass spectrometry is performed in negative ion mode.Various ionization sources, including for example electron impact (EI)ionization, atmospheric pressure chemical ionization (APCI), laser diodethermal desorption (LDTD), or electrospray ionization (ESI), may be usedin embodiments of the present invention. In some embodiments, TBDMS-MMAis ionized by EI in positive ion mode.

One or more separately detectable internal standards may be provided inthe sample prior to treatment of the sample with a derivatizing reagent.In these embodiments, the one or more internal standards may undergoderivatization along with the endogenous MMA, in which case ions of thederivatized internal standards are detected by mass spectrometry.Alternatively, the one or more separately detectable internal standardsmay be provided in the sample after treatment with a derivatizingreagent. The presence or amount of ions generated from the analyte ofinterest may be related to the presence or amount of analyte of interestin the sample by comparison to the amount of ions generated from theinternal standard(s). In some embodiments, the internal standards may beisotopically labeled versions of MMA, such as MMA-²H₃.

Ions detectable in a mass spectrometer may be generated for any internalstandard selected for use. Exemplary spectra generated for TBDMS-MMA-²H₃are discussed in Example 3, and shown in FIGS. 4A-G.

As used herein, an “isotopic label” produces a mass shift in the labeledmolecule relative to the unlabeled molecule when analyzed by massspectrometric techniques. Examples of suitable labels include deuterium(²H), ¹³C, and ¹⁵N. For example, MMA-²H₃ has a mass of about 3 massunits higher than MMA. The isotopic label can be incorporated at one ormore positions in the molecule and one or more kinds of isotopic labelscan be used on the same isotopically labeled molecule.

In other embodiments, the amount of TBDMS-MMA ion or ions may bedetermined by comparison to one or more external reference standards.Exemplary external reference standards include blank plasma or serumspiked with TBDMS-MMA or an isotopically labeled version thereof.External standards typically will undergo the same treatment andanalysis as any other sample to be analyzed.

As used herein, unless otherwise stated, the singular forms “a,” “an,”and “the” include plural reference. Thus, for example, a reference to “aprotein” includes a plurality of protein molecules.

As used herein, the term “purification” or “purifying” does not refer toremoving all materials from the sample other than the analyte(s) ofinterest. Instead, purification refers to a procedure that enriches theamount of one or more analytes of interest relative to other componentsin the sample that may interfere with detection of the analyte ofinterest. Purification of the sample by various means may allow relativereduction of one or more interfering substances, e.g., one or moresubstances that may or may not interfere with the detection of selectedparent or daughter ions by mass spectrometry. Relative reduction as thisterm is used does not require that any substance, present with theanalyte of interest in the material to be purified, is entirely removedby purification.

As used herein, the term “solid phase extraction” or “SPE” refers to aprocess in which a chemical mixture is separated into components as aresult of the affinity of components dissolved or suspended in asolution (i.e., mobile phase) for a solid through or around which thesolution is passed (i.e., solid phase). In some instances, as the mobilephase passes through or around the solid phase, undesired components ofthe mobile phase may be retained by the solid phase resulting in apurification of the analyte in the mobile phase. In other instances, theanalyte may be retained by the solid phase, allowing undesiredcomponents of the mobile phase to pass through or around the solidphase. In these instances, a second mobile phase is then used to elutethe retained analyte off of the solid phase for further processing oranalysis. SPE, including TFLC, may operate via a unitary or mixed modemechanism. Mixed mode mechanisms utilize ion exchange and hydrophobicretention in the same column; for example, the solid phase of amixed-mode SPE column may exhibit strong anion exchange and hydrophobicretention; or may exhibit strong cation exchange and hydrophobicretention.

As used herein, the term “chromatography” refers to a process in which achemical mixture carried by a liquid or gas is separated into componentsas a result of differential distribution of the chemical entities asthey flow around or through a stationary liquid or solid phase.

As used herein, the term “liquid chromatography” or “LC” means a processof selective retardation of one or more components of a fluid solutionas the fluid uniformly percolates through a column of a finely dividedsubstance, or through capillary passageways. The retardation resultsfrom the distribution of the components of the mixture between one ormore stationary phases and the bulk fluid, (i.e., mobile phase), as thisfluid moves relative to the stationary phase(s). Examples of “liquidchromatography” include reverse phase liquid chromatography (RPLC), highperformance liquid chromatography (HPLC), and turbulent flow liquidchromatography (TFLC) (sometimes known as high turbulence liquidchromatography (HTLC) or high throughput liquid chromatography).

As used herein, the term “high performance liquid chromatography” or“HPLC” (sometimes known as “high pressure liquid chromatography”) refersto liquid chromatography in which the degree of separation is increasedby forcing the mobile phase under pressure through a stationary phase,typically a densely packed column.

As used herein, the term “turbulent flow liquid chromatography” or“TFLC” (sometimes known as high turbulence liquid chromatography or highthroughput liquid chromatography) refers to a form of chromatographythat utilizes turbulent flow of the material being assayed through thecolumn packing as the basis for performing the separation. TFLC has beenapplied in the preparation of samples containing two unnamed drugs priorto analysis by mass spectrometry. See, e.g., Zimmer et al., J ChromatogrA 854: 23-35 (1999); see also, U.S. Pat. Nos. 5,968,367, 5,919,368,5,795,469, and 5,772,874, which further explain TFLC. Persons ofordinary skill in the art understand “turbulent flow”. When fluid flowsslowly and smoothly, the flow is called “laminar flow”. For example,fluid moving through an HPLC column at low flow rates is laminar. Inlaminar flow the motion of the particles of fluid is orderly withparticles moving generally in straight lines. At faster velocities, theinertia of the water overcomes fluid frictional forces and turbulentflow results. Fluid not in contact with the irregular boundary “outruns”that which is slowed by friction or deflected by an uneven surface. Whena fluid is flowing turbulently, it flows in eddies and whirls (orvortices), with more “drag” than when the flow is laminar. Manyreferences are available for assisting in determining when fluid flow islaminar or turbulent (e.g., Turbulent Flow Analysis: Measurement andPrediction, P. S. Bernard & J. M. Wallace, John Wiley & Sons, Inc.,(2000); An Introduction to Turbulent Flow, Jean Mathieu & Julian Scott,Cambridge University Press (2001)).

As used herein, the term “gas chromatography” or “GC” refers tochromatography in which the sample mixture is vaporized and injectedinto a stream of carrier gas (such as nitrogen or helium) moving througha column containing a stationary phase composed of a liquid or aparticulate solid and is separated into its component compoundsaccording to the affinity of the compounds for the stationary phase.

As used herein, the term “large particle column” or “extraction column”refers to a chromatography column containing an average particlediameter greater than about 50 μm.

As used herein, the term “analytical column” refers to a chromatographycolumn having sufficient chromatographic plates to effect a separationof materials in a sample that elute from the column sufficient to allowa determination of the presence or amount of an analyte. In a preferredembodiment the analytical column contains particles of about 5 μm indiameter. Such columns are often distinguished from “extractioncolumns”, which have the general purpose of separating or extractingretained material from non-retained materials in order to obtain apurified sample for further analysis.

As used herein, the terms “on-line” and “inline”, for example as used in“on-line automated fashion” or “on-line extraction” refers to aprocedure performed without the need for operator intervention. Incontrast, the term “off-line” as used herein refers to a procedurerequiring manual intervention of an operator. Thus, if samples aresubjected to precipitation, and the supernatants are then manuallyloaded into an autosampler, the precipitation and loading steps areoff-line from the subsequent steps. In various embodiments of themethods, one or more steps may be performed in an on-line automatedfashion.

As used herein, the term “mass spectrometry” or “MS” refers to ananalytical technique to identify compounds by their mass. MS refers tomethods of filtering, detecting, and measuring ions based on theirmass-to-charge ratio, or “m/z”. MS technology generally includes (1)ionizing the compounds to form charged compounds; and (2) detecting themolecular weight of the charged compounds and calculating amass-to-charge ratio. The compounds may be ionized and detected by anysuitable means. A “mass spectrometer” generally includes an ionizer andan ion detector. In general, one or more molecules of interest areionized, and the ions are subsequently introduced into a massspectrometric instrument where, due to a combination of magnetic andelectric fields, the ions follow a path in space that is dependent uponmass (“m”) and charge (“z”). See, e.g., U.S. Pat. No. 6,204,500,entitled “Mass Spectrometry From Surfaces;” U.S. Pat. No. 6,107,623,entitled “Methods and Apparatus for Tandem Mass Spectrometry;” U.S. Pat.No. 6,268,144, entitled “DNA Diagnostics Based On Mass Spectrometry;”U.S. Pat. No. 6,124,137, entitled “Surface-Enhanced PhotolabileAttachment And Release For Desorption And Detection Of Analytes;” Wrightet al., Prostate Cancer and Prostatic Diseases 1999, 2: 264-76; andMerchant and Weinberger, Electrophoresis 2000, 21: 1164-67.

As used herein, the term “operating in negative ion mode” refers tothose mass spectrometry methods where negative ions are generated anddetected. The term “operating in positive ion mode” as used herein,refers to those mass spectrometry methods where positive ions aregenerated and detected.

As used herein, the term “ionization” or “ionizing” refers to theprocess of generating an analyte ion having a net electrical chargeequal to one or more electron units. Negative ions are those having anet negative charge of one or more electron units, while positive ionsare those having a net positive charge of one or more electron units.

As used herein, the term “electron impact” ionization or “EI” ionizationrefers to methods in which an analyte of interest in a gaseous or vaporphase interacts with a flow of electrons. Impact of the electrons withthe analyte produces analyte ions, which may then be subjected to a massspectrometry technique.

As used herein, the term “chemical ionization” or “CI” refers to methodsin which a reagent gas (e.g. ammonia) is subjected to electron impact,and analyte ions are formed by the interaction of reagent gas ions andanalyte molecules.

As used herein, the term “fast atom bombardment” or “FAB” refers tomethods in which a beam of high energy atoms (often Xe or Ar) impacts anon-volatile sample, desorbing and ionizing molecules contained in thesample. Test samples are dissolved in a viscous liquid matrix such asglycerol, thioglycerol, m-nitrobenzyl alcohol, 18-crown-6 crown ether,2-nitrophenyloctyl ether, sulfolane, diethanolamine, andtriethanolamine. The choice of an appropriate matrix for a compound orsample is an empirical process.

As used herein, the term “matrix-assisted laser desorption ionization”or “MALDI” refers to methods in which a non-volatile sample is exposedto laser irradiation, which desorbs and ionizes analytes in the sampleby various ionization pathways, including photo-ionization, protonation,deprotonation, and cluster decay. For MALDI, the sample is mixed with anenergy-absorbing matrix, which facilitates desorption of analytemolecules.

As used herein, the term “surface enhanced laser desorption ionization”or “SELDI” refers to another method in which a non-volatile sample isexposed to laser irradiation, which desorbs and ionizes analytes in thesample by various ionization pathways, including photo-ionization,protonation, deprotonation, and cluster decay. For SELDI, the sample istypically bound to a surface that preferentially retains one or moreanalytes of interest. As in MALDI, this process may also employ anenergy-absorbing material to facilitate ionization.

As used herein, the term “electrospray ionization” or “ESI,” refers tomethods in which a solution is passed along a short length of capillarytube, to the end of which is applied a high positive or negativeelectric potential. Solution reaching the end of the tube is vaporized(nebulized) into a jet or spray of very small droplets of solution insolvent vapor. This mist of droplets flows through an evaporationchamber, which is heated slightly to prevent condensation and toevaporate solvent. As the droplets get smaller the electrical surfacecharge density increases until such time that the natural repulsionbetween like charges causes ions as well as neutral molecules to bereleased.

As used herein, the term “atmospheric pressure chemical ionization” or“APCI,” refers to mass spectrometry methods that are similar to ESI;however, APCI produces ions by ion-molecule reactions that occur withina plasma at atmospheric pressure. The plasma is maintained by anelectric discharge between the spray capillary and a counter electrode.Then ions are typically extracted into the mass analyzer by use of a setof differentially pumped skimmer stages. A counterflow of dry andpreheated N2 gas may be used to improve removal of solvent. Thegas-phase ionization in APCI can be more effective than ESI foranalyzing less-polar species.

The term “atmospheric pressure photoionization” or “APPI” as used hereinrefers to the form of mass spectrometry where the mechanism for thephotoionization of molecule M is photon absorption and electron ejectionto form the molecular ion M+. Because the photon energy typically isjust above the ionization potential, the molecular ion is lesssusceptible to dissociation. In many cases it may be possible to analyzesamples without the need for chromatography, thus saving significanttime and expense. In the presence of water vapor or protic solvents, themolecular ion can extract H to form MH+. This tends to occur if M has ahigh proton affinity. This does not affect quantitation accuracy becausethe sum of M+ and MH+ is constant. Drug compounds in protic solvents areusually observed as MH+, whereas nonpolar compounds such as naphthaleneor testosterone usually form M+. See, e.g., Robb et al., Anal. Chem.2000, 72(15): 3653-3659.

As used herein, the term “inductively coupled plasma” or “ICP” refers tomethods in which a sample interacts with a partially ionized gas at asufficiently high temperature such that most elements are atomized andionized.

As used herein, the term “field desorption” refers to methods in which anon-volatile test sample is placed on an ionization surface, and anintense electric field is used to generate analyte ions.

As used herein, the term “desorption” refers to the removal of ananalyte from a surface and/or the entry of an analyte into a gaseousphase. Laser diode thermal desorption (LDTD) is a technique wherein asample containing the analyte is thermally desorbed into the gas phaseby a laser pulse. The laser hits the back of a specially made 96-wellplate with a metal base. The laser pulse heats the base and the heatcauses the sample to transfer into the gas phase. The gas phase samplemay then be drawn into an ionization source, where the gas phase sampleis ionized in preparation for analysis in the mass spectrometer. Whenusing LDTD, ionization of the gas phase sample may be accomplished byany suitable technique known in the art, such as by ionization with acorona discharge (for example by APCI).

As used herein, the term “selective ion monitoring” is a detection modefor a mass spectrometric instrument in which only ions within arelatively narrow mass range, typically about one mass unit, aredetected.

As used herein, “multiple reaction mode,” sometimes known as “selectedreaction monitoring,” is a detection mode for a mass spectrometricinstrument in which a precursor ion and one or more fragment ions areselectively detected.

As used herein, an “amount” of an analyte in a body fluid sample refersgenerally to an absolute value reflecting the mass of the analytedetectable in volume of sample. However, an amount also contemplates arelative amount in comparison to another analyte amount. For example, anamount of an analyte in a sample can be an amount which is greater thana control or normal level of the analyte normally present in the sample.

The term “about” as used herein in reference to quantitativemeasurements not including the measurement of the mass of an ion, refersto the indicated value plus or minus 10%. Mass spectrometry instrumentscan vary slightly in determining the mass of a given analyte. The term“about” in the context of the mass of an ion or the mass/charge ratio ofan ion refers to +/−0.50 atomic mass unit.

The summary of the invention described above is non-limiting and otherfeatures and advantages of the invention will be apparent from thefollowing detailed description of the invention, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows exemplary chromatogram for TBDMS-MMA. FIG. 1B showsexemplary chromatogram for TBDMS-MMA-²H₃ (internal standard). Detailsare discussed in Example 3.

FIG. 2 shows an exemplary calibration curve for MMA determined bymethods described in Example 3.

FIGS. 3A-3G show exemplary product ion spectra (covering the m/z rangeof about 50 to 310) for fragmentation of the TBDMS-MMA precursor ionwith m/z of about 289.0 at various collision energies. FIG. 3A showsproduct ion spectra for CID at 3. FIG. 3B shows product ion spectra forCID at 5. FIG. 3C shows product ion spectra for CID at 8. FIG. 3D showsproduct ion spectra for CID at 10. FIG. 3E shows product ion spectra forCID at 12. FIG. 3F shows product ion spectra for CID at 15. FIG. 3Gshows product ion spectra for CID at 18. Details are described inExample 3.

FIGS. 4A-4G show exemplary product ion spectra (covering the m/z rangeof about 60 to 300) for fragmentation of the TBDMS-MMA-²H₃ precursor ionwith m/z of about 292.0 at various collision energies. FIG. 4A showsproduct ion spectra for CID at 3. FIG. 4B shows product ion spectra forCID at 5. FIG. 4C shows product ion spectra for CID at 8. FIG. 4D showsproduct ion spectra for CID at 10. FIG. 4E shows product ion spectra forCID at 12.

FIG. 4F shows product ion spectra for CID at 15. FIG. 4G shows production spectra for CID at 18. Details are described in Example 3.

FIG. 5 shows exemplary data collected for TBDMS-MMA and TBDMS-MMA-²H₃for the m/z transitions at 289→189 (at 3 V) and 292→189 (at 3 V),respectively. Details are discussed in Example 3.

FIG. 6 shows exemplary data collected for TBDMS-MMA and TBDMS-MMA-²H₃for the m/z transitions at 289→147 (at 10 V) and 292→147 (at 10 V),respectively. Details are discussed in Example 3.

FIG. 7 shows a plot of coefficient of variation (CV) versusconcentration for quantitation of TBDMS-MMA in serum measured bymonitoring the m/z transition at 289→189 (at 3 V). Details are discussedin Example 5.

FIG. 8 shows a plot demonstrating the linearity of response forquantitation of TBDMS-MMA in serum measured by monitoring the m/ztransition at 289→189 (at 3 V). Details are discussed in Example 6.

FIGS. 9A and 9B show plots for method comparison between GC-MS/MS andLC-MS/MS. FIG. 9A shows a plot of values obtained from the same sampleby each method, while FIG. 9B shows the differences between obtainedvalues as a function of MMA concentration. Details are discussed inExample 8.

DETAILED DESCRIPTION OF THE INVENTION

Methods are described for determining the amount of MMA in a sample.More specifically, tandem mass spectrometric methods are described fordetecting and quantifying tert-butyldimethylsilyl derivatized MMA(TBDMS-MMA) in a sample. Thus, the amount of MMA in a sample may bedetermined by derivatizing MMA in a sample to form TBDMS-MMA, measuringthe amount of TBDMS-MMA in the sample by tandem mass spectrometry, andrelating the amount of TBDMS-MMA measured to the amount of MMA in thesample. In some embodiments, TBDMS-MMA derivatives may be prepared byderivatization of MMA with methyl-(t-butyldimethylsilyl)trifluoroacetamide (MTBSFTA).

The methods may use a chromatography technique, such as gaschromatography (GC), to perform a purification of derivatized MMA,combined with methods of mass spectrometry (MS), thereby providing ahigh-throughput assay system for detecting and quantifying MMA in asample. Preferred embodiments are particularly well suited forapplication in large clinical laboratories for automated MMAquantitation.

Suitable test samples for use in methods of the present inventioninclude any test sample that may contain the analyte of interest. Insome preferred embodiments, a sample is a biological sample; that is, asample obtained from any biological source, such as an animal, a cellculture, an organ culture, etc. In certain preferred embodiments,samples are obtained from a mammalian animal, such as a dog, cat, horse,etc. Particularly preferred mammalian animals are primates, mostpreferably male or female humans. Preferred samples comprise bodilyfluids such as blood, plasma, serum, saliva, cerebrospinal fluid, ortissue samples; preferably plasma (including EDTA and heparin plasma)and serum; most preferably serum. Such samples may be obtained, forexample, from a patient; that is, a living person, male or female,presenting oneself in a clinical setting for diagnosis, prognosis, ortreatment of a disease or condition.

The present invention also contemplates kits for a MMA quantitationassay. A kit for a MMA quantitation assay may include a kit comprisingthe compositions provided herein. For example, a kit may includepackaging material and measured amounts of derivatizing reagent forgeneration of TBDMS-MMA derivatives (such as MTBSFTA) and anisotopically labeled internal standard, in amounts sufficient for atleast one assay. Typically, the kits will also include instructionsrecorded in a tangible form (e.g., contained on paper or an electronicmedium) for using the packaged reagents for use in a MMA quantitationassay.

Calibration and QC pools for use in embodiments of the present inventionare preferably prepared using a matrix similar to the intended samplematrix.

As described above, elevated MMA levels may be caused in a patient by avitamin B₁₂ deficiency. Therefore, some embodiments of the presentinvention may be used to diagnose vitamin B₁₂ deficiency in a patient,or monitor compliance with and/or efficacy of treatment of vitamin B₁₂deficiency in a patient. For example, detection of MMA levels in excessof about [[223±20 nMol/L (Please Confirm)]] are indicative of vitaminB₁₂ deficiency.

Sample Preparation for Mass Spectrometric Analysis

In preparation for mass spectrometric analysis, MMA may be enrichedrelative to one or more other components in the sample (e.g. protein) byvarious methods known in the art, including for example, liquidchromatography, filtration, centrifugation, thin layer chromatography(TLC), electrophoresis including capillary electrophoresis, affinityseparations including immunoaffinity separations, extraction methodsincluding ethyl acetate or methanol extraction, and the use ofchaotropic agents or any combination of the above or the like.

Protein precipitation is one method of preparing a test sample,especially a biological test sample, such as serum or plasma. Proteinpurification methods are well known in the art, for example, Polson etal., Journal of Chromatography B 2003, 785:263-275, describes proteinprecipitation techniques suitable for use in methods of the presentinvention. Protein precipitation may be used to remove most of theprotein from the sample leaving MMA in the supernatant. The samples maybe centrifuged to separate the liquid supernatant from the precipitatedproteins; alternatively the samples may be filtered to removeprecipitated proteins. The purified MMA may then be derivatized withreagent capable of generating TBDMS-MMA, preferably MTBSFTA or anisotopically labeled variant thereof.

Another method of sample purification that may be used prior to massspectrometry is liquid chromatography (LC). Certain methods of liquidchromatography, including HPLC, rely on relatively slow, laminar flowtechnology. Traditional HPLC analysis relies on column packing in whichlaminar flow of the sample through the column is the basis forseparation of the analyte of interest from the sample. The skilledartisan will understand that separation in such columns is a diffusionalprocess and may select LC, including HPLC, instruments and columns thatare suitable for use with derivatized vitamin D. The chromatographiccolumn typically includes a medium (i.e., a packing material) tofacilitate separation of chemical moieties (i.e., fractionation). Themedium may include minute particles, or may include a monolithicmaterial with porous channels. A surface of the medium typicallyincludes a bonded surface that interacts with the various chemicalmoieties to facilitate separation of the chemical moieties. One suitablebonded surface is a hydrophobic bonded surface such as an alkyl bonded,cyano bonded surface, or highly pure silica surface. Alkyl bondedsurfaces may include C-4, C-8, C-12, or C-18 bonded alkyl groups. Thechromatographic column includes an inlet port for receiving a sample andan outlet port for discharging an effluent that includes thefractionated sample. The sample may be supplied to the inlet portdirectly, or from an extraction column, such as an on-line SPE cartridgeor a TFLC extraction column.

Certain methods of liquid chromatography, including HPLC, rely onrelatively slow, laminar flow technology. Traditional HPLC analysisrelies on column packing in which laminar flow of the sample through thecolumn is the basis for separation of the analyte of interest from thesample. The skilled artisan will understand that separation in suchcolumns is a diffusional process and may select LC, including HPLC,instruments and columns that are suitable for use with derivatized MMA.The chromatographic column typically includes a medium (i.e., a packingmaterial) to facilitate separation of chemical moieties (i.e.,fractionation). The medium may include minute particles, or may includea monolithic material with porous channels. A surface of the mediumtypically includes a bonded surface that interacts with the variouschemical moieties to facilitate separation of the chemical moieties. Onesuitable bonded surface is a hydrophobic bonded surface such as an alkylbonded, cyano bonded surface, or highly pure silica surface. Alkylbonded surfaces may include C-4, C-8, C-12, or C-18 bonded alkyl groups.The chromatographic column includes an inlet port for receiving a sampleand an outlet port for discharging an effluent that includes thefractionated sample. The sample may be supplied to the inlet portdirectly, or from an extraction column, such as an on-line SPE cartridgeor a TFLC extraction column.

In one embodiment, the sample may be applied to the LC column at theinlet port, eluted with a solvent or solvent mixture, and discharged atthe outlet port. Different solvent modes may be selected for eluting theanalyte(s) of interest. For example, liquid chromatography may beperformed using a gradient mode, an isocratic mode, or a polytyptic(i.e. mixed) mode. During chromatography, the separation of materials iseffected by variables such as choice of eluent (also known as a “mobilephase”), elution mode, gradient conditions, temperature, etc.

In certain embodiments, an analyte may be purified by applying a sampleto a column under conditions where the analyte of interest is reversiblyretained by the column packing material, while one or more othermaterials are not retained (i.e., solid phase extraction). In theseembodiments, a first mobile phase condition can be employed where theanalyte of interest is retained by the column, and a second mobile phasecondition can subsequently be employed to remove retained material fromthe column, once the non-retained materials are washed through.Alternatively, an analyte may be purified by applying a sample to acolumn under mobile phase conditions where the analyte of interestelutes at a differential rate in comparison to one or more othermaterials. Such procedures may enrich the amount of one or more analytesof interest relative to one or more other components of the sample.

By careful selection of valves and connector plumbing, two or morechromatography columns may be connected as needed such that material ispassed from one to the next without the need for any manual steps. Inpreferred embodiments, the selection of valves and plumbing iscontrolled by a computer pre-programmed to perform the necessary steps.Most preferably, the chromatography system is also connected in such anon-line fashion to the detector system, e.g., an MS system. Thus, anoperator may place a tray of samples in an autosampler, and theremaining operations are performed under computer control, resulting inpurification and analysis of all samples selected.

Another method of sample purification that may be used prior to massspectrometry is gas chromatography (GC) (also sometimes known asvapor-phase chromatography or liquid partition chromatography). Gaschromatography may be employed in methods of analyzing compounds thatcan be vaporized without decomposition. In gas chromatography, themobile phase is a carrier gas, usually an inert gas such as helium or anunreactive gas such as nitrogen. The stationary phase is a microscopiclayer of liquid or polymer on an inert solid support. The stationaryphase is contained inside a column. The gaseous compounds being analyzedinteract with the walls of the column, which may be coated with variousstationary phases. Varying degrees of interaction of each gaseouscompound with the stationary phase cause each compound to elute at adifferent times. Thus, for a given stationary phase and given columnconditions, each compound will have a characteristic retention time. Insome embodiments, a GC column comprising cyanopropyl methyl siloxane,such as an Agilent DB-23 cis/trans FAME GC column, is used to purifyTBDMS-MMA. In some embodiments, the carrier gas is helium. In someembodiments, the column is heated to a temperature of about 185° C. andheld for about 0.4 minutes, then the temperature is ramped to about 300°C.; for example at a rate of about 85° C./minute. Once the temperaturereaches about 300° C., the temperature is then held constant, forexample for about 0.6 minutes.

In some embodiments, an extraction column may be used for purificationof MMA prior to mass spectrometry. In such embodiments, MMA is purifiedby applying a first mobile phase (containing the analyte) to anextraction column which captures the analyte, and then eluting thecaptured analyte with a second mobile phase. The eluent may be collectedand chromatographed on a second extraction column or on an analyticalcolumn (such as a GC column) prior to ionization. In some embodiments,extraction of MMA via extraction column is done before MMA isderivatized; alternatively, MMA may be derivatized prior to extraction.

Detection and Quantitation by Mass Spectrometry

In various embodiments, derivatized MMA may be ionized by any methodknown to the skilled artisan. Mass spectrometry is performed using amass spectrometer, which includes an ion source for ionizing thefractionated sample and creating charged molecules for further analysis.For example ionization of the sample may be performed by electron impact(EI) ionization, chemical ionization, electrospray ionization (ESI),photon ionization, atmospheric pressure chemical ionization (APCI),photoionization, atmospheric pressure photoionization (APPI), Laserdiode thermal desorption (LDTD), fast atom bombardment (FAB), liquidsecondary ionization (LSI), matrix assisted laser desorption ionization(MALDI), field ionization, field desorption, thermospray/plasmasprayionization, surface enhanced laser desorption ionization (SELDI),inductively coupled plasma (ICP) and particle beam ionization. Theskilled artisan will understand that the choice of ionization method maybe determined based on the analyte to be measured, type of sample, thetype of detector, the choice of positive versus negative mode, etc.

Derivatized MMA may be ionized in positive or negative mode. Inpreferred embodiments, derivatized MMA is ionized by electron impact(EI) ionization in positive mode.

In mass spectrometry techniques generally, after the sample has beenionized, the positively or negatively charged ions thereby created maybe analyzed to determine a mass-to-charge ratio. Suitable analyzers fordetermining mass-to-charge ratios include quadrupole analyzers, iontraps analyzers, and time-of-flight analyzers. Exemplary ion trapmethods are described in Bartolucci, et al., Rapid Commun. MassSpectrom. 2000, 14:967-73.

The ions may be detected using several detection modes. For example,selected ions may be detected, i.e. using a selective ion monitoringmode (SIM), or alternatively, mass transitions resulting from collisioninduced dissociation or neutral loss may be monitored, e.g., multiplereaction monitoring (MRM) or selected reaction monitoring (SRM).Preferably, the mass-to-charge ratio is determined using a quadrupoleanalyzer. For example, in a “quadrupole” or “quadrupole ion trap”instrument, ions in an oscillating radio frequency field experience aforce proportional to the DC potential applied between electrodes, theamplitude of the RF signal, and the mass/charge ratio. The voltage andamplitude may be selected so that only ions having a particularmass/charge ratio travel the length of the quadrupole, while all otherions are deflected. Thus, quadrupole instruments may act as both a “massfilter” and as a “mass detector” for the ions injected into theinstrument.

One may enhance the resolution of the MS technique by employing “tandemmass spectrometry,” or “MS/MS”. In this technique, a precursor ion (alsocalled a parent ion) generated from a molecule of interest can befiltered in an MS instrument, and the precursor ion subsequentlyfragmented to yield one or more fragment ions (also called daughter ionsor product ions) that are then analyzed in a second MS procedure. Bycareful selection of precursor ions, only ions produced by certainanalytes are passed to the fragmentation chamber, where collisions withatoms of an inert gas produce the fragment ions. Because both theprecursor and fragment ions are produced in a reproducible fashion undera given set of ionization/fragmentation conditions, the MS/MS techniquemay provide an extremely powerful analytical tool. For example, thecombination of filtration/fragmentation may be used to eliminateinterfering substances, and may be particularly useful in complexsamples, such as biological samples.

Alternate modes of operating a tandem mass spectrometric instrumentinclude product ion scanning and precursor ion scanning. For adescription of these modes of operation, see, e.g., E. Michael Thurman,et al., Chromatographic-Mass Spectrometric Food Analysis for TraceDetermination of Pesticide Residues, Chapter 8 (Amadeo R.Fernandez-Alba, ed., Elsevier 2005) (387).

The results of an analyte assay may be related to the amount of theanalyte in the original sample by numerous methods known in the art. Forexample, given that sampling and analysis parameters are carefullycontrolled, the relative abundance of a given ion may be compared to atable that converts that relative abundance to an absolute amount of theoriginal molecule. Alternatively, external standards may be run with thesamples, and a standard curve constructed based on ions generated fromthose standards. Using such a standard curve, the relative abundance ofa given ion may be converted into an absolute amount of the originalmolecule. In certain embodiments, an internal standard is used togenerate a standard curve for calculating the quantity of MMA. Methodsof generating and using such standard curves are well known in the artand one of ordinary skill is capable of selecting an appropriateinternal standard. For example, in some embodiments an isotopicallylabeled MMA (e.g., MMA-²H₃) may be used as an internal standard.Numerous other methods for relating the amount of an ion to the amountof the original molecule will be well known to those of ordinary skillin the art.

One or more steps of the methods may be performed using automatedmachines. In certain embodiments, one or more purification steps areperformed on-line, and more preferably all of the purification and massspectrometry steps may be performed in an on-line fashion.

In certain embodiments, such as MS/MS, where precursor ions are isolatedfor further fragmentation, collision activated dissociation (CAD) isoften used to generate fragment ions for further detection. In CAD,precursor ions gain energy through collisions with an inert gas, andsubsequently fragment by a process referred to as “unimoleculardecomposition.” Sufficient energy must be deposited in the precursor ionso that certain bonds within the ion can be broken due to increasedvibrational energy.

In some embodiments, MMA in a sample is detected and/or quantified usingMS/MS as follows. MMA is first extracted from a sample with a solidphase extraction column. Then, MMA in the purified sample is derivatizedwith a reagent capable of generating TBDMS-MMA, preferably MTBSFTA. Thepurified samples (now comprising TBDMS-MMA) are then subjected to gaschromatography; the flow of analyte from the chromatographic columnenters an EI ionization source and TBDMS-MMA ions are generated. TheTBDMS-MMA ions, e.g. precursor ions, pass through the orifice of theinstrument and enter the first quadrupole. Quadrupoles 1 and 3 (Q1 andQ3) are mass filters, allowing selection of ions (i.e., selection of“precursor” and “fragment” ions in Q1 and Q3, respectively) based ontheir mass to charge ratio (m/z). Quadrupole 2 (Q2) is the collisioncell, where ions are fragmented. The first quadrupole of the massspectrometer (Q1) selects for TBDMS-MMA precursor ions, which areallowed to pass into the collision chamber (Q2), while unwanted ionswith any other mass/charge ratio collide with the sides of thequadrupole and are eliminated. Precursor ions entering Q2 collide with acollision gas and fragment. The fragment ions generated are passed intoquadrupole 3 (Q3), where certain TBDMS-MMA fragment ions are selectedwhile other ions are eliminated.

As ions collide with the detector they produce a pulse of electrons thatare converted to a digital signal. The acquired data is relayed to acomputer, which plots counts of the ions collected versus time. Theresulting mass chromatograms are similar to chromatograms generated intraditional GC-MS methods. The areas under the peaks corresponding toparticular ions, or the amplitude of such peaks, may be measured andcorrelated to the amount of the analyte of interest. In certainembodiments, the area under the curves, or amplitude of the peaks, forfragment ion(s) and/or precursor ions are measured to determine theamount of MMA. As described above, the relative abundance of a given ionmay be converted into an absolute amount of the original analyte usingcalibration standard curves based on peaks of one or more ions of aninternal molecular standard.

EXAMPLES Example 1: Solid Phase Extraction and TBDMS Derivatization

The following extraction techniques were conducted on serum samples.

MMA was extracted from serum samples by solid phase extraction (SPE) ona strong anion exchange (SAX) packed SPE column. Water and 100% methanolwere used as wash and elution solvents, respectively. Alternatively, MMAwas extracted from serum samples by SPE on a SAX packed SPE column wherewater and 3 M acetic acid/methanol were used as wash and elutionsolvents, respectively.

MMA was then derivatized by adding 150 μl of MTBSTFA with 1% tert-butyldimethylchlorosilane (TBDMCS) in hexane or pyridine to the collectedextracted samples. The resulting mixtures were heated to about 70° C.for 15-20 minutes and chilled at about 4° C. for about 10 minutes. Aftercooling, the mixtures were transferred to GC/MS vials for analysis.

Alternatively, MMA from the sample was derivatized by first evaporatingthe eluted samples to dryness, adding 500 μl of 20% triethylamine inmethanol to the dried samples, drying the samples again, adding 100 μlMTBSTFA with 1% tert-butyl dimethylchlorosilane (TBDMCS) in hexane orpyridine to the dried samples, and vortexing for 2-3 minutes. Theresulting mixtures were heated to about 60° C. for about 30 minutes andthen chilled. After cooling, the mixtures were then subjected toGC-MS/MS analysis.

Example 2: Purification of TBDMS-MMA Derivatives with Gas Chromatography

Purification of TBDMS-MMA derivatives with gas chromatography wasconducted with an Agilent 7890A Fast GC equipped with a cyanopropylmethyl siloxane analytical column, such as an Agilent DB-23 cis/transFAME GC column (DB-23: J&W 122-2361, Agilent Technologies, Wilmington,Del. USA). The GC column was heated to a temperature of about 185° C.and held for about 0.4 minutes. The GC column was then heated further toa temperature of about 300° C. at a rate of about 85° C./minute. Oncethe temperature reached about 300° C., the temperature was held at about300° C. for about 0.6 minutes. Helium was used as the carrier gas atapproximately 3 ml/min. Under these conditions, retention time forTBDMS-MMA was about 1.76-1.78 minutes. Succinic acid, a potentiallyinterfering species with a similar mass to MMA, was clearly separatedfrom MMA under this procedure with a retention time of approximately2.12 minutes.

Alternatively, purification of TBDMS-MMA derivatives with gaschromatography was conducted with an Agilent 7890 GC equipped with acyanopropyl methyl siloxane analytical column, such as an Agilent DB-23cis/trans FAME GC column (DB-23: J&W 122-2361, Agilent Technologies,Wilmington, Del. USA). The GC column was heated to a temperature ofabout 300° C. Helium was used as the carrier gas at approximately 4ml/min. Under these conditions, retention time for TBDMS-MMA was about1.30 minutes.

Example 3: Detection and Quantitation of TBDMS-MMA Derivatives by MS/MS

MS/MS was performed on the above generated samples using a Agilent 7001GC/MS Triple Quadrupole MS system (Agilent Technologies, Wilmington,Del. USA). TBDMS-MMA exiting the analytical column flowed to theelectron impaction ionization interface of the MS/MS analyzer andionized.

Ions passed to the first quadrupole (Q1), which selected TBDMS-MMAprecursor ions with a mass-to-charge ratio of 289.0±0.5 m/z. Ionsentering quadrupole 2 (Q2) collided with argon gas to generate ionfragments, which were passed to quadrupole 3 (Q3) for further selection.Simultaneously, the same process using isotope dilution massspectrometry was carried out with internal standard, TBDMS-MMA-²H₃. Masstransitions observed at collision energies of 3V and 10 V (positivepolarity) are shown in Table 1.

TABLE 2 Mass Transitions for TBDMS-MMA and TBDMS-MMA- ²H₃ (internalstandard) (Positive Polarity) Product Ions Product Ions Precursor Ion(m/z) (m/z) Analyte (m/z) (CE = 3 V) (CE = 10 V) TBDMS-MMA 289.0 ± 0.5189.0 ± 0.5 147.0 ± 0.5 147.0 ± 0.5  73.0 ± 0.5  73.0 ± 0.5TBDMS-MMA-²H₃ 292.0 ± 0.5 189.0 ± 0.5 147.0 ± 0.5 147.0 ± 0.5  73.0 ±0.5  73.0 ± 0.5

Exemplary chromatograms for TBDMS-MMA and TBDMS-MMA-²H₃ (internalstandard), are shown in FIGS. 1A and 1B, respectively.

An exemplary calibration curve for the determination of TBDMS-MMA inserum as measured by monitoring the transition from 289.0±0.5→147.0±0.5at a collision energy of about 10 V is shown in FIG. 2.

Example 3: Exemplary Spectra from MS/MS Analysis of TBDMS-MMA

Tandem mass spectrometric analyses of TBDMS-MMA and TBDMS-MMA-²H₃ wereconducted by selecting precursor ions with m/z of about 289.0 and 292.0,respectively. The precursor ions were fragmented at collision energiesof 3, 5, 8, 10, 12, 15, and 18 V. Exemplary product ion scans generatedfrom these analyses are presented in FIGS. 3A-3G and 4A-4G for TBDMS-MMAand TBDMS-MMA-²H₃, respectively.

Exemplary MRM transitions for the quantitation of TBDMS-MMA includefragmenting a precursor ion with a m/z of about 289.0±0.5 to one or moreproduct ions selected from the group of ions with a m/z of about189.0±0.5, 147.0±0.5, and 73.0±0.5. As seen in FIGS. 3A-3G and 4A-4G,selection of fragments to monitor for quantitation may depend on thecollision energy selected. At a higher collision energy, such as 10 V,fragment ions may include ions with m/z of about 147.0±0.5 and 73.0±0.5,while at lower collision energies, such as about 3 V, fragment ions mayalso include ions with m/z of about 189.0±0.5. Similarly, MRMtransitions for the quantitation of TBDMS-MMA-²H₃ include fragmenting aprecursor ion with a m/z of about 292.0 to one or more product ionsselected from the group of ions with a m/z of about 189.0±0.5,147.0±0.5, and 73.0±0.5. Again, at a higher collision energy, such as 10V, fragment ions may include ions with m/z of about 147.0±0.5 and73.0±0.5, while at lower collision energies, such as about 3 V, fragmentions may also include ions with m/z of about 189.0±0.5.

Data was collected for quantitation of TBDMS-MMA and TBDMS-MMA-²H₃ inserum by monitoring MRM transitions generated by collision energies of 3V and 10 V. Exemplary data from the 3 V collision energy (TBDMS-MMA:289.0→189.0; and TBDMS-MMA-²H₃: 292.0→189.0) are shown in FIG. 5, whileexemplary data from the 10 V collision energy (TBDMS-MMA: 289.0→147.0;and TBDMS-MMA-²H₃: 292.0→147.0) are shown in FIG. 6.

Example 4: Within Run and Total Precision Studies

For within run and total precision studies, Low, Mid, and Highconcentration control pools were prepared with target values of 150,1000, and 5000 nMol/L MMA by spiking charcoal stripped serum with MMA inwater. MMA was quantitated in these serum samples as described above, bymonitoring the MRM transition from a MRM precursor ion with m/z of about289.0±0.5 to a fragment ion with m/z of about 189.0±0.5 (collisionenergy about 3 V).

For the within run precision studies, aliquots of the Low, Mid, and Highcontrol pools were analyzed 10 times each in a single sample run. Thecoefficients of variation (CV) from each concentration were used todetermine if reproducibility within run was acceptable. Statisticalanalysis of the results determined that the CV for the Low, Mid, andHigh pools were about 2.0%, 2.7%, and 2.2%, respectively. In addition,standard deviations (SD) for each of the pools were calculated to beabout 3 nMol/L, 28 nMol/L, and 111 nMol/L, respectively. Data from theseexperiments are presented in Table 3.

TABLE 3 Within Run Precision Control Pool Low Mid High (150 nMol/L)(1000 nMol/L) (5000 nMol/L) Replicate Measured MMA Concentration(nMol/L) 1 153 1054 4960 2 153 1033 5181 3 156 1028 4984 4 157 997 49525 151 1070 4961 6 154 1037 5191 7 149 1011 4855 8 159 1028 5021 9 1551090 4965 10 154 1021 5130 Mean 154 1037 5020 Std Dev 3 28 111 CV 2.0%2.7% 2.2%

For the total precision studies, aliquots of the Low, Mid, and Highcontrol pools were analyzed on 19 separate days. On 17 of those days,the control pools were run in duplicate, while on two of the days, thecontrols were run in singleton. A total of 36 data points for eachcontrol pool were generated and statistically analyzed. The CV for theLow, Mid, and High pools were calculated to be about 10.9%, 9.3%, and8.0%, respectively. In addition, SD for each of the pools werecalculated to be about 19 nMol/L, 104 nMol/L, and 420 nMol/L,respectively. Data from these experiments are presented in Table 4.

TABLE 4 Total Precision Control Pool Low Mid High (150 nMol/L) (1000nMol/L) (5000 nMol/L) Replicate Measured MMA Concentration (nMol/L) A153 1054 4960 B 154 1021 5130 A 150 1026 4731 B 148 966 4765 A 156 10565294 B 147 977 5061 A 149 1074 5279 B 181 1173 5341 A 186 1184 5684 B177 1020 5059 A 169 1139 5158 B 181 1141 4634 A 180 1182 5164 B 157 10214616 A 178 1179 5382 B 156 1021 4743 A 164 1132 5280 B 151 1125 5423 A184 1209 5805 B 185 1155 5224 A 191 1097 5461 B 180 1182 4924 A 168 9875127 B 193 1197 4912 A 171 964 4672 A 163 1093 5044 B 174 1464 5903 A200 1200 5269 B 175 — — A 180 1088 5603 B 147 1170 5660 A 175 1125 4600B 141 1143 5573 A 223 1261 6400 B 197 1273 5526 A 203 1252 5876 B 1631236 5677 Count (n) 37 36 36 Mean 172 1127 5249 Std Dev 19 104 420 CV10.9% 9.3% 8.0%

Example 5: Analytical Sensitivity: Limit of Blank (LOB), Limit ofDetection (LOD), and Lower Limit of Quantitation (LLOQ)

The Limit of Blank (LOB) was determined by analyzing a zero calibrator(i.e., a blank of stripped serum) 20 times in a single run. The LOB wascalculated to be about 8 nMol/L.

The Limit of Detection (LOD) is the point at which a value is beyond theuncertainty associated with its measurement and is defined as fourstandard deviations from the zero concentration. To determine the LOD, ablank of stripped serum was analyzed 20 times in a single sample run,and the mean and SD were determined. The resulting LOD was calculated tobe about 9 mMol/L.

The Lower Limit of Quantitation (LLOQ) is the point where measurementsbecome quantitatively meaningful. Analyte response at the LLOQ isidentifiable, discrete and reproducible with a precision (i.e.,coefficient of variation (CV)) of less than 20% and an accuracy of 80%to 120%. The LLOQ was determined by assaying five different strippedserum samples spiked with MMA at about 6 nMol/L, 13 nMol/L, 25 nMol/L,50 nMol/L, and 100 nMol/L in 5 replicates over 5 days, according to themethod described above in Example 4. CV's were calculated for eachlevel. The LLOQ was calculated from the data to be 9 mMol/L. Data fromanalysis of the lower three concentrations and a blank are presented inTable 5. The graphical representations of CV versus concentration isshown in FIG. 7.

TABLE 5 Lower Limit of Quantitation Experiments MMA Concentration(nMol/L) Run # Result # 6 mMol/L 13 mMol/L 25 mMol/L 0 mMol/L 1 1 8.0009.000 25.000 6.760 2 8.000 10.000 21.000 8.562 3 8.000 12.000 19.0007.110 4 8.000 14.000 25.000 6.492 5 4.000 13.000 26.000 6.232 2 1 8.00011.000 25.000 7.243 2 7.000 13.000 20.000 6.208 3 5.000 13.000 24.0006.684 4 6.000 13.000 22.000 6.794 5 7.000 12.000 23.000 7.661 3 1 4.00010.000 25.000 6.339 2 5.000 10.000 24.000 6.901 3 5.000 12.000 20.0006.329 4 4.000 10.000 19.000 6.967 5 5.000 11.000 26.000 7.082 4 1 7.00016.000 28.000 6.811 2 6.000 12.000 29.000 6.994 3 7.000 13.000 25.0007.578 4 6.000 12.000 20.000 7.127 5 7.000 12.000 26.000 7.003 5 1 8.00015.000 31.000 — 2 8.000 15.000 29.000 — 3 8.000 16.000 26.000 — 4 9.00012.000 26.000 — 5 8.000 15.000 26.000 — Count (n) 25 25 25 20 Mean 6.64012.440 24.400 6.944 SD 1.524 1.938 3.240 0.555 CV 23.1% 15.6% 12.9% 8.3%

Example 6: Reportable Range and Linearity

A sample with high concentration of MMA (8868 nMol/L) was mixed with asample with a low concentration of MMA (99 nMol/L) in differentproportions to achieve five samples with concentrations across therange. The five samples were prepared with the following proportions100% Low (expected concentration of 99 nMol/L), 75% Low/25% High(expected concentration of 2291 nMol/L), 50% Low/50% High (expectedconcentration of 4484 nMol/L), 25% Low/75% High (expected concentrationof 6676 nMol/L), and 100% High (expected concentration of 8868 nMol/L).Analysis of samples at these five concentrations as described in Example4 demonstrated linear response across a reportable range of up to atleast about 8870 nMol/L. A graphical representation demonstrating thelinearity of response is shown in FIG. 8.

Example 7: Recovery Studies

Six patient serum pools (STD 1-6) were spiked with a known amount ofMMA. Samples 1-3 (500 μL) were spiked with 50 μL of 3333 nMol/L MMAstandard in stripped serum, and samples 4-6 (500 μL) were spiked with 50μL of 10,000 nMol/L MMA standard in stripped serum. Non-spiked sampleswere prepared for comparison by adding 50 μL of blank stripped serum to500 μL of each of Samples 1-6. For each serum pool, 2 baseline (blankserum added) and 4 spiked replicates were tested as described in Example4. The amount recovered was calculated as the difference between themean of the spiked and baseline samples. Percent recovery was calculatedto be the ratio of the amount recovered (observed) divided by the amountadded. The six pools yielded an average accuracy of about 104.1%. Allassays were within the acceptable accuracy range of 85-115%. The resultsof the spiked specimen recovery studies are presented in Table 6.

TABLE 6 Spiked Specimen Recovery Studies STD 1 STD 2 STD 3 STD 4 STD 5STD 6 Measured Concentration (nMol/L) Blank 399.77 420.96 431.60 1070.501113.17 1478.64 Result 1 421.41 433.23 463.68 1111.00 1169.87 1689.27Result 2 434.17 440.31 449.28 1142.52 1160.13 1533.30 Result 3 420.86445.55 461.15 1158.69 1193.30 1576.17 Result 4 414.25 422.68 429.291107.55 1132.14 1555.99 Spiked Mean 418.09 432.55 447.00 1118.05 1153.721566.67 % Recovery 104.58 102.75 103.57 104.44 103.64 105.95

Example 8: Method Comparison Studies

The performance of the GC-MS/MS assay described in Example 4 wascompared against quantitation of MMA by LC-MS/MS. Forty patient serumsamples were analyzed in four individual runs over four days by eachmethod. Difference and % difference were calculated for each patientsample. The results of these studies are shown plotted in FIGS. 9A(GC-MS/MS vs LC-MS/MS) and 9B (difference vs amount detected).

Example 9: Hemolysis, Lipemia, and Icteria Studies

The effect hemolysis, lipemia, and icteria have on the assay was alsoinvestigated.

Hemolysis. The effect of hemolysis was evaluated by preparing fivepatient serum pools as follows. A hemolysate was prepared by washing 2mL of heparinized whole blood three times with phosphate buffered saline(PBS). After the final wash, the cells were frozen at about −70° C. forabout 30 minutes. The hemolyzed cell mixture was removed from thefreezer and mixed by mechanical vortex. For each of the five patientserum pools, 200 μL of hemolysate was added to 2 mL of serum. A baseline(non-hemolytic) sample was prepared for each pool by the addition of 200μL of PBS to 2 mL of serum. For each pool, baseline and hemolyticsamples were analyzed in quadruplicate and the differences between meanand hemolyzed samples observed. Results of analysis of all pools wereacceptable (i.e., between 85% and 115% of baseline). Results of thesestudies are presented in Table 7.

Lipemia. The effect of lipemia was evaluated by preparing five patientserum pools each with 200 μL of Intralipid added to 2 mL of serum. Abaseline (non-lipemic) sample was prepared for each pool by the additionof 200 μL of PBS to 2 mL of serum. For each pool, baseline and lipemicsamples were analyzed in quadruplicate and the differences between meanand lipemic samples observed. Results of analysis of all pools wereacceptable (i.e., between 85% and 115% of baseline). Results of thesestudies are presented in Table 7.

Icteria. The effect of icteria was evaluated by preparing five patientserum pools each with 200 μL of Verichem High Bilirubin Standard Level F(30 mg/dL total and direct bilirubin) added to 2 mL of serum. A baseline(non-icteric) sample was prepared for each pool by the addition of 200μL of PBS to 2 mL of serum. For each pool, baseline and icteric sampleswere analyzed in quadruplicate and the differences between mean andicteric samples observed. Results of analysis of four of the five poolswere unacceptable (i.e., above 115% of baseline). Results of thesestudies are presented in Table 7.

TABLE 7 Hemolysis, Lipemia, and Bilirubin Interference Studies Pool 1Pool 2 Pool 3 Pool 4 Pool 5 Average MMA Concentration (nMol/L) Baseline210 187 170 199 150 Hemolysis 209 193 174 204 157 Icterus 203 188 174198 152 Lipemia 223 235 219 238 195 % Recovery Hemolysis 100% 103% 102%103% 105% Icterus  97%  97% 100%  97%  97% Lipemia 110% 125% 126% 120%128%

Example 7: MMA Reference Intervals

Serum samples from 30 healthy male and 30 healthy female donors (totaln=60) were assayed for MMA as described above, and results statisticallyanalyzed to determine reference ranges for healthy individuals. Healthyreference ranges were determined to be within about 67 nMol/L to about223 nMol/L. No sex difference was observed. Raw data from these studiesis presented in Table 8.

TABLE 8 Reference Interval Studies MMA Sample (nMol/L) Below RI Above RI1 137.00 2 166.00 3 67.00 4 100.00 5 153.00 6 80.00 7 136.00 8 71.00 961.00 YES 10 111.00 11 96.00 12 129.00 13 101.00 14 161.00 15 95.00 16197.00 17 169.00 18 203.00 19 121.00 20 113.00 21 125.00 22 126.00 23277.00 YES 24 122.00 25 124.00 26 160.00 27 84.00 28 144.00 29 120.00 30104.00 31 96.00 32 98.00 33 99.00 34 109.00 35 163.00 36 106.00 37156.00 38 111.00 39 120.00 40 143.00 41 134.00 42 159.00 43 76.00 44101.00 45 230.00 YES RI Lower Limit 67 RI Upper Limit 223 Number ofdonors 45 Number above RI 2 Percent above RI 4.4% Number below RI 1Percent below RI 2.2% Percent outside RI 6.7%

The contents of the articles, patents, and patent applications, and allother documents and electronically available information mentioned orcited herein, are hereby incorporated by reference in their entirety tothe same extent as if each individual publication was specifically andindividually indicated to be incorporated by reference. Applicantsreserve the right to physically incorporate into this application anyand all materials and information from any such articles, patents,patent applications, or other physical and electronic documents.

The methods illustratively described herein may suitably be practiced inthe absence of any element or elements, limitation or limitations, notspecifically disclosed herein. Thus, for example, the terms“comprising”, “including,” containing”, etc. shall be read expansivelyand without limitation. Additionally, the terms and expressions employedherein have been used as terms of description and not of limitation, andthere is no intention in the use of such terms and expressions ofexcluding any equivalents of the features shown and described orportions thereof. It is recognized that various modifications arepossible within the scope of the invention claimed. Thus, it should beunderstood that although the present invention has been specificallydisclosed by preferred embodiments and optional features, modificationand variation of the invention embodied therein herein disclosed may beresorted to by those skilled in the art, and that such modifications andvariations are considered to be within the scope of this invention.

The invention has been described broadly and generically herein. Each ofthe narrower species and subgeneric groupings falling within the genericdisclosure also form part of the methods. This includes the genericdescription of the methods with a proviso or negative limitationremoving any subject matter from the genus, regardless of whether or notthe excised material is specifically recited herein.

Other embodiments are within the following claims. In addition, wherefeatures or aspects of the methods are described in terms of Markushgroups, those skilled in the art will recognize that the invention isalso thereby described in terms of any individual member or subgroup ofmembers of the Markush group.

That which is claimed is:
 1. A method for determining the amount ofmethylmalonic acid (MMA) in a sample by tandem mass spectrometry, themethod comprising: (a) adding a deuterated internal standard to a samplecontaining MMA; (b) purifying MMA by solid phase extraction (SPE)followed by liquid chromatography; (c) ionizing MMA in negative ion modeto generate one or more ions detectable by mass spectrometry; and (d)determining the amount of said one or more ions by tandem massspectrometry; wherein the amount of ions determined in step (d) isrelated to the amount of MMA originally present in the sample.
 2. Themethod of claim 1, wherein said ions comprise a precursor ion with amass to charge ratio of 289.0±0.5 and one or more fragment ions selectedfrom the group consisting of fragment ions with mass to charge ratios(m/z) of 189.0±0.5, 147.0±0.5, and 73.0±0.5.
 3. The method of claim 1,wherein said sample comprises plasma or serum.
 4. The method of claim 1,wherein said fragment ion comprises an ion with m/z of 189.0±0.5.
 5. Themethod of claim 1, wherein said fragment ion comprises an ion with m/zof 147.0±0.5.